Monitors for sensing and recording physiologic signals are often used to record more than a single physiologic signal. Sometimes, it happens that two physiologic signals of the same type are recorded on the same recorder or display. Such is, for example, the case if a fetal monitor is used to monitor the heart rates of two fetuses, as in the case of twins (it may also well be that two fetuses from different mothers are recorded with the same fetal monitor).
As one specific example of the problems associated with recording of multiple physiologic signals of the same type, the case of a fetal monitor will be discussed in more detail herein. However, the present invention is not limited to such monitors.
A fetal monitor picks up and records two parameters of particular importance for the assessment of fetal well-being, namely the fetal beat-to-beat heart rate and maternal labor. Various techniques are available for recording these parameters.
The best indication of the fetal heart rate may be obtained by a so-called fetal scalp electrode which is screwed into the fetal epidermis. The electrocardiogram obtained from this electrode is used to determine the fetal beat-to-beat heart rate as the inverse of the time interval between two subsequent beats.
Unfortunately, this kind of measurement (also called "direct ECG") may only be used after rupture of the membranes. Prior to that point in time, indirect methods have to be used. The most common one is the ultrasound technique, wherein ultrasound waves are transmitted in the direction of the fetal heart. The reflected ultrasound wave is subject to Doppler shift caused by the moving parts of the fetal heart, e.g., the heart walls or heart valves. The Doppler shift can be extracted from the received ultrasound wave by means of appropriate demodulation, and the inverse of the time interval between subsequent peaks in the Doppler signal reveals the beat-to-beat heart rate. Autocorrelation is a method which is particularly useful to detect the peaks in the Doppler signal.
Likewise, there are external and internal methods for recording maternal labor. The external transducers are applied to the maternal abdomen and contain usually tension-sensitive resistance elements; the intravaginal transducer is a simple pressure transducer.
Fetal monitors of this kind are typically stand-alone units, i.e., they contain the transducer electronics, the necessary hardware for processing the received signals, a display and a recorder. The recorder--such as a thermal printer--prints on a paper strip which contains two preprinted scalings, one for the fetal heart rate, and the other for maternal labor.
Some fetal monitors have meanwhile also the capability to record twins, i.e., to record the heart rate of two fetuses, by means of appropriate transducers. However, recording these two heart rates produces a problem of considerable clinical importance.
Due to the limited space available for the recorder, standard paper is used for twin monitoring. Thus, the two heart rate traces have to be printed on the same preprinted grid or scale. If the obstetrician is monitoring twins with similar or approximately equal heart rates, the two traces will therefore overlap, such that they cannot easily be distinguished, and partially cover each other. This is a serious disadvantage in clinical practice, as the beat-to-beat heart rate traces contain relatively fast spectral components (in the range of 0.5 Hz to 0.01 Hz). These spectral components--usually referred to as "variability"--contain valuable diagnostic information which may be lost when two heart rate traces overlap.
It is therefore a strong desire to increase the readability of two (or more) fetal heart rate traces in case they are recorded on the same grid of a printer (or another kind of display).
In this context, it has to be noted that only one of one hundred births concerns twins. Therefore, the provision of another printer--for example, with broader paper for an additional grid--is no acceptable solution. In contrast, it is the regular goal of the design of fetal monitors not to create a significant overhead for the rare cases of twin births.
There have already been multiple attempts in the prior art to overcome the underlying problem. One could, for example, record the two heart rate traces with lines of different thickness, or different patterns. Although this solution is undoubtedly advantageous, the variability may still be covered by the overlapping traces; for example, the thicker trace does not allow to detect reliably the components of higher frequency in the other trace.
As an alternative, one could also provide a printer with the capability to print colored traces. However, this solution suffers from two other disadvantages. The first is that a colored printer will considerably increase manufacturing expenses, and thus the cost of the monitor, and that it is more difficult to operate (for example, the colored tapes have to be exchanged from time to time). It is thus obviously not justified to spend a colored printer simply for one percent of twin births.
The other disadvantage results from clinical practice. The obstetrician, as well as the midwife are acquainted with the usual appearance of trace records and will resist to use a different technology. It has also to be taken into consideration that a different appearance of the heart rate traces may cause errors in their interpretation, such that dangerous situations of the fetus may be lost.
By the way, the latter considerations also apply to several further prior art solutions which will be discussed below. Clinical personnel is acquainted with the present appearance of fetal heart rate records, and any change of this appearance will hardly be accepted, and may give rise to diagnostic errors. Thus, it is an important demand for all fetal monitors, even in the case of twin monitoring, to provide a record which is basically similar to the records with which clinical personnel is acquainted.
Still another solution of the underlying problem is to provide recorder paper with an additional heart rate scale or grid. For reasons already discussed above, such paper cannot be broader than the paper used for a single fetus, as no extra recorder can be provided for the case of twin monitoring. Thus, the two heart rate scales provide a smaller scale factor than usual.
This latter solution is unacceptable in clinical practice. First, and as already discussed above, the outer appearance (namely the scale factor, as well as the geometric arrangement of the heart rate traces) is different from usual recordings, which may cause errors in diagnostic interpretation. For example, the midwife is acquainted with a certain geometry of a deceleration in the fetal heart rate trace. If such a deceleration is printed with a different scale factor, the decrease of the heart rate may not appear as strong as in a usual record, such that a fetal stress situation may not be detected. It has further to be taken into account that a reduced scale factor also reduces the resolution, such that heart rate variability is more difficult to detect.
One further problem associated with double scales is that different paper has to be used for the case of twin monitoring, as compared to the monitoring of a single fetus. It would obviously not be appropriate to record a single fetus on a paper with two heart rate grids, as one of the grids would not be used in this case. Therefore, the recorder paper had to be changed from time to time, which makes handling more difficult.
For the above reasons, the provision of a paper with two different heart rate grids is also not acceptable.
One has tried to avoid these drawbacks by the provision of recorder paper wherein two different scalings are printed in the same grid (an example will be discussed in the detailed description). However, this solution has also not been accepted in clinical practice. One reason therefore is that at least one of the fetal heart rate traces is printed with a different scale factor, which implies the drawbacks already discussed above. Another reason is that it is quite difficult to assign the two traces to the correct scaling. This problem has also legal aspects, as an incorrect assignment may give rise to liability questions.
One could also use the usual paper with a single heart rate grid for the case of twin monitoring and print a second, different scale with the built-in recorder. This solution would avoid the need to change the recorder paper. However, it would not solve the problem of heart rates printed in different scale factors than usual. Further, the second scale would be printed in the same color as the heart rate traces themselves, such that the traces, and the second grid, cannot easily be distinguished (usually, the grid is preprinted on the recorder paper in a color different from the heart rate traces; for example, the grid may be preprinted in green or red, and the recorder may record the heart rate traces in black).
Another disadvantage associated with the latter solution is that the recorder is considerably stressed if it has to print a second grid, such that its lifetime is reduced. If the recorder is a thermal printer, printing of vertical grid lines may also cause the paper to stick against the printing head. This effect, in turn, produces noise which is undesirable in the delivery room.
The latter disadvantages may also apply if not only one grid, but both grids are printed by the built-in recorder. Although the provision of white paper would offer the opportunity to print different grids in the case of a single fetus, as compared to twins, the recorder would be over-stressed, and the problem of a different scale factor would likewise not be solved.
The above considerations show that there have been a multiplicity of attempts to solve the problems associated with twin monitoring, but that none of them provided an acceptable solution. Thus, there is a continuing need for an improved method for recording fetal heart rates originating from different fetuses.